Accept all valid reasoning item content formats in multi-turn input:
- Array of objects: [{"type":"reasoning_text","text":"..."}] (spec format)
- Plain string: "thinking about it" (OpenCode format)
- Null: content:null with encrypted_content (Codex, openai/codex#11834)
- Omitted entirely: no content field present
Previously threw "item['content'] is not an array" for non-array formats,
breaking OpenCode multi-turn conversations. The encrypted_content field
is accepted but ignored for local models (no server-side decryption).
Add 4 tests covering each format variant.
Refs: openai/codex#11834, anomalyco/opencode#19081
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